Auto-RX & FP mileage went up then back down

[ALS]

I had Auto-Rx in the engine and added FP for my 900 plus mile trip.
I took the 97 Volvo 960 out yesterday and headed for up state NY to see a guy about parts and to put some miles on the Auto-Rx cleaning cycle. While I was at it I filled the tank and added 4 oz of FP to my fresh 21 gallons. Well the first 520 miles the gas mileage jumped from a normal 26.3 mpg to 28.8 miles per gallon. I was totally thrilled and filled her back up (18 gallons) and added another 4 oz of FP and headed back home. When I got home my gas mileage had dropped back to a normal 26.25 miles per gallon. Same route same everything but the fuel mileage dropped. I used the same gas (Sunoco 89) in both fill ups.
Would the computer compensate for the cleaned up fuel system? I couldn't tell if there was a difference in performance since it already ran well before both additives were introduced to the car. The 26.3 was what the car got on the highway before the Auto-Rx and the Fuel power were added.

 

[Quest]

mostly due to differnt energy contents in your gas (different batches, even though brand name and grade are the same) and also driving behaviours.

 

[Bill]

ALS,

Did you drive down hill on the way there and up hill on the way back?

 

[W.T.McGlynn]

Did you have a head wind on the return tip ?
That would do it.

 

[Ugly3]

Some physical reason, not ARX. Temp, wind, rain, etc.

 

[ALS]

Everything was the same including the cruise control being set at 65 mph all the way up and back. A/C was on up and back and the outside temperature was within 10 degrees from start to finish. I had a light rain in the first 20 minutes out of 7 hours of driving up to my destination. Other than those 20 minutes I had perfect weather the rest of the trip. There was little or no wind that I could tell.

 

[LarryL]

A one or two tank measurement will have small sample errors, gas levels, weather, wind, grades etc. It's difficult over a short term. That's why every gas mileage device always works at first, you will find what you are looking for, most times. You just need more samples over a much longer period of time.

 

[JHZR2]

if your engine was in a decent state of cleanliness and tune, neither of these items would necessarily have any effect.

That has been my experience in 6 different vehicles that I regularly use/own.

JMH

 

[V1]

Even a cooler outside air temperature can affect MPG's. Cooler tires = less pressure = lower MPG.

I agree that the best way to calculate mileage improvements is over several tanks/miles, and to try and keep those variables you have control over as constant as possible. It doesn't take much to tip the scale.

Use microshaft XL spreadsheet - you can easily set up some formulas that will figure out mileage for you. Just plug and chug the numbers after each fillup.